The membrane is a sheet-like structure with thickness of about 6-10 nm, mainly composed of lipids . They are relatively small molecules having both hydrophilic and hydrophobic parts. Hydrophobic regions of lipid molecules face each other while their hydrophilic parts are exposed outward. This leads to the formation of a so-called lipid bilayer. In such a bilayer, regardless of extracellular or intracellular sides, proteins are attached in a random fashion without any particular pattern (Fig. 2.6).
Proteins either can be attached to a surface of a lipid bilayer or can span one or more times through cell membrane. The orientation of proteins is asymmetric—proteins on one side of a lipid bilayer are different from those attached to the other side. On the extracellular side, lipids and proteins are glycosylated, i.e., they possess covalently attached oligosaccharide structures called glycans. Inside the cell, proteins are frequently linked to cytoskeleton facing the cytosol. It is important to underline that both proteins and lipids are free to laterally move in a plain of a bilayer but movement from one bilayer side to another is strongly restricted.
All membranes have similar basic structure and certain common functions; however, each type of membrane present within a cell interior plays certain distinctive role due to a unique set of proteins and lipid composition. For example, neurons possesses so-called myelin sheath, which is a plasma membrane wrapped several times around a cell. The myelin sheath acts as a passive electric insulator because of its high content of lipids. Distinct composition of membranes in mitochondria is crucial for enzyme-catalyzed processes, and thus, these membranes have more proteins than lipids .
Figure 2.6 Scheme of a fluid mosaic model for cell membrane.